Method of manufacturing lithium ferrite magnetic cores



United wastage f ABSTRACT OF THE DISCLOSURE A lithium ferrite core suitable as magnetic memory element having a rectangularnysteresis loop and a compositron corresponding to Li Fe O where (x-|-3y) is between 7.8 and 8.0, x/y is between 0.19 and 0.22 and z is between 3.9 and 4. The core has an outer diameter not exceeding 0.9 mm. and inner diameter of half the outer diameter. The switching time of the core does not exceed 0.7 pLSCC.

Our invention relates to an annular magnetic core suited for use as a magnetic memory element, and to a method of manufacturing such a magnetic core. More particularly, our invention relates to a magnetic core consisting essentially of a ferrite having a substantially rectangular hysteresis loop.

As is known magnetic memory elements are generally used at present in electronic computers. Their suitability for this use is determined by the pulse or dynamic characteristics of such memory elements. In this connection, it is important that there be a marked difference between the maximum value uVl of the undisturbed one-signal and the maximum value dVz of the disturbed zero-signal. (It is known that in a high-grade storage element, the value uVl and the value rVl, that is to say, the maximum value of the disturbed one-signal, differ only slightly.)

In addition, with a given rise time of the control current the time interval between the beginning of the control current pulse and the instant at which the output voltage of the one-signal reaches its maximum value, must have a substantially constant value within a wide temperature range. For practical reasons the starting time is not taken as the beginning of the control current pulse, but at the instant at which the control current reaches a strength of 10% of its maximum value. The term peak time (T of a magnetic core is, in this case, to be understood to mean the time interval between the instant at which the control current reaches a strength of 10% of its maximum value and the instant at which the output voltage of the one-signal produced by the particular control current pulse has become a maximum. Obviously this peak time depends upon the rise time (T,) of the control current pulse.

Further, in this connection, another important pulse characteristic is the switching time, T The shorter this switching time is, the faster the storage element is. Consequently, the general tendency is to obtain the lowest possible values of the switching time.

Hitherto, variations in the current pulse characteristics of storage elements which were due to temperature variations have, in most cases, been corrected by varying the strength of the control current. Alternatively, whole systems of memory elements have been placed in a thermo- 3,413,228 atented Nov. 25, 1968 statically controlled oven to prevent disturbing temperature variations. However, these methods are complicated and cumbersome. Furthermore, they are useless if, during operation of the system, temperature differences occur between the individual storage elements because one element is switched a greater number of times in a given period of time than the other or others. It is, therefore, of great importance that there should be available storage elements which not only have a sufficiently large squareness ratio, but in which also the output voltage of the one-signal and the peak time (T depend only to a slight extent, if at all, on the temperature within a wide temperature range.

It is a principal object of our invention to provide a magnetic memory element which has markedly improved pulse characteristics which are substantially constant over a Wide temperature range.

A further object of our invention is to provide a magnetic memory element having a large difference between the maximum value uVl of the undisturbed one-signal and the maximum value dVz of the disturbed zero-signal.

A still further object of our invention is to provide a magnetic memory element in which the rise time of a control current pulse is 0.1 microsecond.

Another object of our invention is to provide a magnetic memory element in which the switching time for a control pulse is less than 0.7 microsecond.

And still another object of our invention is to provide a magnetic memory element in which the output voltage and the peak time are substantially independent of temperature over a wide temperature range.

And yet another object of our invention is to provide a method of manufacturing a magnetic memory element having improved pulse characteristics which are substantially constant over a wide temperature range.

These and further objects of the invention will appear as the specification progresses.

The magnetic memory cores, according to the invention, consist essentially of materials having a spinel structure and a composition corresponding substantially to the formula Li Fe O in which x, y and z must satisfy the following conditions: 7.8g(x+3y) 58.0 0.19gx/y5022 3.9gy/zg4.0

It has been further found that cores, according to our invention, preferably should have an outer diameter not exceeding 0.9 mm. and an inner diameter of at least onehalf the outer diameter. In order to obtain the desired results, the cores have to be made in accordance with the method described further on in this specification. Such cores, it has been found, exhibit at least the following properties. The switching time for such a core does not exceed 0.7 microsecond. The value of the quotient uVl/dVz is greater than 4.5 with a disturbance ratio of 0.61. The temperature coefiicient for uVl does not exceed 0.7% per C., and the temperature coefficient for T does not-exceed 0.03% per C., both being applicable between 0 and C. In the same temperature range, the temperature coefficient for dVz is negligibly small while that of T, (which is less critical) is about equal to that of T A magnetic core, according to the invention, can be Li O.5Fe O allowance being made for'the fact that dur- L ing heating a slight amount of lithium may evaporate. After being cooled, the prefired initial mixture is finelydivided and pressed into rings of the specified dimensions. The resulting product is heated to a temperature between 1275 C. and 1330 C. in air or a mixture of air and oxygen on a supporting surface made of a refractory metal or a refractory metal alloy within a period of time of 90 seconds. The latter temperature is maintained for 4 to 12 minutes and subsequently the sintered product is cooled to a temperature between 875 C. and 1030 C. at a rate of most C. per minute. The sintered product is th n rapidly cooled by bringing it into contact with air or an air-oxygen mixture at room temperature.

The following examples are illustrative of the invention:

EXAMPLE I A mixture of 16.7 mol. percent of finely-divided lithium carbonate, Li CO and 83.3 mol. percent of finelydivided iron oxide, Fe O was prefired at a temperature of 550 C. for two hours. After the prefired product had been cooled, it was finely-divided and compressed to form rings. These rings were heated to a temperature of 1300 C. in air on a supporting surface consisting of platinum or a platinum-rhodium alloy in an electric furnace in a period or" time of 60 seconds, held at that temperature for 10 minutes, and then cooled in and together with the furnace to 950 C. in a period of time of minutes. The cores were subsequently taken from the furnace and quenched in contact with air of room temperature.

The outer diameter of the resulting sintered bodies was 0.820 mm. and their inner diameter 0.500 mm. The pulse characteristics are specified in the table following Example III.

EXAMPLE II A mixture of 16.3 mol. percent of finely-divided lithium carbonate, Li CO and 83.7 mol. percent of finely-divided iron oxide, F6 0 was prefired at 750 C. for two hours. The prefired product was cooled, finely-divided and then compressed to form rings. These rings were heated to a temperature of 1282 C. in air on a supporting surface of platinum or a platinum-rhodium alloy in an electric furnace in a period of time of seconds, held at that temperature for 10 minutes, then cooled to 980 C. in and together with the furnace. Finally, the rings were quenched in contact with air of room temperature.

The outer diameter and the inner diameter of the resulting sintered bodies were equal to those of the sintered bodies obtained according to Example I. The pulse characteristics are specified in the table following Example III.

EXAMPLE III A mixture of 17.9 mol. percent of finely-divided lithium carbonate, Li CO and 82.1 mol. percent of finely-divided iron oxide, Fe O was prefired at 750 C. for two hours. The prefired product was cooled, finely-divided and then compressed to form rings. These rings were heated to a temperature of 1300 C. in air on a supporting surface of platinum or a platinum-rhodium alloy in an electric furnace of a peroid of time of 45 seconds, held at the said temperature for 10 minutes, then'cooled to 900 C. in and with the aid of the furnace in a period of minutes. Subsequently, the rings were taken from the furnace and quenched in contact with air of room temperature.

The outer and inner diameters of the resulting sintered bodies were equal to those of the sintered bodies manu- 4 factured according to Example I. The pulse characteristics are specified in the following table.

Example I Example II Example 111 Control current (ma) G50 650 650 T (micro seconds)" 0. 280 0. 310 0. 310 '1, (micro seconds) 0. 1 0. 1 O. 1 '1', (micro scconds) 0. (1'00 0 610 0. 610

rVl (mv.) 35 31 34 dVz (rm-2) 5.1 4.8 5. 5 Quotient uVl/dVz 6.8 6.1 6. 2

All above measurements were carried out at a temperature of 40 (J.

Temperature coefficient 01 uVl (in percent per (J.) 0.5 0. 63 0.53 In temperature range C.)

from 0-80 0-80 0-100 Temperature cocttieient of Tp e a": 0d l) rccut per C.) 0.02 0.02 0.02 In temperature range C.)

from 0-80 0-80 0-100 It will be apreciated that the foregoing examples are illustrative only and that variations therein are within the scope of the invention which is defined in the appended claims. We, thereofore, do not wish to be limited to these examples but desire that the claims be constructed as broadly as possible in view of the art.

What is claimed is:

1. A method of manufacturing an annular magnetic core having an outer diameter not exceeding 0.9 mm., an inner diameter of at least one-half the outer diameter, a switching time not exceeding 0.7 microsecond, a value of the quotient uVl/dVz of 4.5 with a disturbance ratio of 0.61, a temperature coefficient for uVl not exceeding 0.7% per C. and a temperature coefficient for T not exceeding 0.03% per C., both of said temperature coefiicients. being applicable within the temperature range from 0 C to C., said core consisting essentially of a material having a composition corresponding to the formula Li Fe O where: (x-t-Sy) is greater than 7.8 and less than 8.0, x/y is greater than 0.19 and less than 0.22, and z is between 3.9 and 4.0 comprising the steps, mixing in finely-divided form about 16 to 18 mol percent of U 0 and about 82 to 84 mol percent of Fe O heating the mixture to a temperature of about 500 C. to 700 C. to prefire the same, finely-dividing the prefired mixture, compressing the finely-divided prefired mixture into annular rings having the specified dimensions, heating the rings to a temperature between about 1275 C. and 1330 C. in an oxidizing atmosphere containing at least as much oxygen as air on a refractory metal support in about seconds, maintaining said rings at said temperature for about 4 to 12 minutes, cooling the rings from said latter temperature to a temperature between 875 C. and 1030 C. at a rate of about 30 C. per minute, and quenching said rings by contacting them with an atmosphere containing at least as much oxygen as air at room temperature.

2. A method as defined in claim 1 in which the refractory metal support consists of platinum.

3. A method as defined in claim 1 in which the refractory metal support is an alloy of platinum and rhodium.

References Cited UNITED STATES PATENTS 3,093,588 6/1963 Brown 252-62.61 3,226,328 12/1965 Esveldt et al. 25262.61 3,293,184 12/1966 Van Driel et al. 25262.61 

